THE X-15 PROGRAM IN RETROSPECT

Implications for Future Research Aircraft



A large new area of potential interest for exploration by future research aircraft has been opened up by the advent of hypersonic air breathing propulsion systems. These systems, which were not generally believed feasible in the early years of the X-15 program, employ some combination of the ramjet or scramjet for cruise, with the turbojet or rocket for acceleration and climb. The systems of interest include space launch vehicles, military strike or reconnaissance vehicles, and commercial transports, with speeds extending upward to about Mach 12. Many new problems not covered by X-15 research are found in the propulsion system, cryogenic fuel tankage, lightweight radiation-cooled structures, and in the piloting and operations areas. While it is not clear at present that a new air-breathing research airplane system will prove justifiable, it is desirable to explore and evaluate some of the possibilities.

If we look to the X-15 experience as a guide, what prime features does it suggest? What features were most vital to the role it has played in aerospace history? Clearly the decision in 1954 to proceed quickly with a general research tool as opposed to a configuration fully optimized with respect to the 1954 vision of the feature mission was vital. If the optimized pseudo-prototype route had been followed one can see with the advantage of hindsight that we would have picked the wrong mission, the wrong structure, the wrong aerodynamic shapes, and the wrong propulsion.

More important, in the 3 or 4 years which all of this optimization might have consumed we would have dissipated our technology lead time, and with the start of the space age in 1958, in all probability the X-15 would never have been attempted. A second basic feature of the X-15 that proved vital was the design of the system with great latitude in performance so that it would reach well beyond the hypersonic aerodynamic corridor into simulated space flight.

Figure 20 - airbreathing research airplane
Figure 20. Airbreathing research airplane study configuration.


A possible new research airplane system conforming to these and other X-15 guidelines is illustrated in figure 20. (Footnote: This is one of several systems receiving preliminary study in the United States.) It is a lifting-body cruise configuration designed for Mach numbers up to 12. Its acceleration engine is a hydrogen fueled J-2S rocket adapted from an upper-stage engine of the Saturn vehicle. Integrated into the lower surface is a research scramjet engine sized to paver the airplane in cruise. Following guidelines from our X-15 experience the vehicle is kept as small as possible, aboub 25 meters in length, and it remains in the Mach 12 environment only long enough for research purposes, about 5 minutes. As we have leaxned from the X-15, a new hypersonic research airplane system is likely to have a long lifetime of perhaps 15 years, during which many new unsuspected ideas for research and changes in configuration are likely to appear as the program develops. Accordingly, we are proposing here actually three different vehicle arrangements. We would start the program with the lifting-body rocket glider without the air-breathing research engine. Later, a delta-winged version using the same subsystems would be flown. And, finally, the integrated scranjet research engine shown here would be installed. Provision for structural cooling schemes including direct fuel cooling, air-film cooling, and other schemes likely to appear in these vehicles might also be made.

The case for approval of such a new research airplane is no longer simple as it was in 1954. A major difference is the high level of confidence now enjoyed by the partial-simulation techniques of ground-based research and development. The X-15 program itself, together with the successfully developed reentry vehicle systems, has thus tended to eliminate a major justification vhich existed in 1954. In the author's opinion no new exploratory research airplane can ever again be successfully promoted primarily on the grounds that it will produce unique flight data, without which a successful technology cannot be achieved. Thus there is not likely to be a future research airplane unless a high valuation is placed on the other vital but less tangible contributicns - the focusing of the countless detailed efforts in many areas, the revelation of new and unsuspected problems for research, the overall stimulation of technology development, and the early availability of new technology for important but initially unforeseeable new applications. The X-15 experience affirms that the ex ploratory research airplane is a most effective device for producing these values.


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